Frog-Killing Plague

The mossy red-eyed frog (Duellmanohyla soralia) is one of hundreds of species threatened by a virulent fungus that may be responsible for 90 extinctions in the past 50 years.Jonathan E. Kolby/Honduras Amphibian Rescue & Conservation Center.

The Plague Killing Frogs Everywhere Is Far Worse Than Scientists Thought

As a threat to wildlife, an amphibian fungus has become “the most deadly pathogen known to science.”

In 2019, 41 scientists published the first worldwide analysis of a fungal outbreak that’s been wiping out frogs for decades. The devastation turns out to be far worse than anyone had previously realized.

Florida's native green treefrog (Hyla cinerea) is no match in size to exotic Cuban Tree Frogs or Cane Toads.

Amphibian fungal panzootic causes catastrophic and ongoing loss of biodiversity

Writing in the journal Science, the researchers conclude that populations of more than 500 species of amphibians have declined significantly because of the outbreak—including at least 90 species presumed to have gone extinct. The figure is more than twice as large as earlier estimates.

“That’s fairly seismic,” said Wendy Palen, a biologist at Simon Fraser University who is a co-author of A deadly amphibian disease goes global a commentary accompanying the study. “It now earns the moniker of the most deadly pathogen known to science.”

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Cuban Tree Frogs (Osteopilus septentrionalis) have invaded Florida, brought in by travel and trade. The frogs brought pathogens with them, and they eat native tree frogs. Photo: Phillip Lott

Trade to Blame?

Dr. Palen says in part that "[t]rade routes have shaped human history by connecting distant civilizations, allowing the exchange of materials, technology, and people, but also diseases. Pathogens have been frequent hitchhikers, bringing them in contact with new hosts that provide the fuel for epidemic outbreaks of disease. Humans are not the only victims of trade-driven diseases, but scientists have only recently begun to appreciate the risk to biodiversity of inadvertently introducing new pathogens to naïve evolutionary arenas."

Scientists first noticed in the 1970s that some frog populations were declining quickly; by the 1980s, some species appeared to be extinct. The losses were puzzling, because the frogs were living in pristine habitats, unharmed by pollution or deforestation.

Chytridiomycosis causes amphibian mortality associated with population declines in the rain forests of Australia and Central America

In the late 1990s, researchers discovered that frogs in both Australia and Panama were infected with a deadly fungus, which they named Batrachochytrium dendrobatidis — Bd, for short. They found that Bd resulted resulting in the disease chytridiomycosis.

An Oriental fire-bellied toad (Bombina orientalis), which was imported into Europe from South Korea. Photo: Frank Pasmans

The fungus turned up in other countries, but studies of its DNA suggest that Bd originated on the Korean Peninsula. In Asia, amphibians seem impervious to Bd, but when it got to other parts of the world—probably via the international trade in pet amphibians—the pathogen reached hundreds of vulnerable species.

Amphibians are infected with Bd by contact with other animals or by spores floating in the water. The fungus invades skin cells and multiplies. An infected frog’s skin will start to peel away as the animal grows sluggish. Before it dies, a frog may manage to hop its way to a new stream or pond, spreading the fungus further.

A Cuban Tree Frog warms itself on a cool Florida night, with Christmas lights.

Photo: James Snyder.

Spread of Chytridiomycosis Has Caused the Rapid Global Decline and Extinction of Frogs

In 2007, researchers speculated that Bd might be responsible for all known declines of frogs that had no other apparent cause—about 200 species. For the most part, however, scientists studied Bd at the local level, looking at its impacts on particular species in particular places.

“We knew that frogs were dying all around the world, but no one had gone back to the start and actually assessed what the impact was,” said Benjamin Scheele, an ecologist at Australian National University and the lead author of the new study.

In 2015, Dr. Scheele and his colleagues gathered data from over 1,000 published papers on Bd, and traveled around the world to meet with experts and hear their unpublished observations.

Not only did the team analyze data on living amphibians, but they also looked at data from museums, where scientists found Bd DNA embedded in preserved specimens tucked away in cabinets.

The new study showed that some amphibians are at greater risk than others.

The fungus thrives in cool, moist conditions. As a result, frogs that live in cloud forests on mountainsides have been hit particularly hard.

Espada's marsupial frog, near the Gocta Waterfall in the Chachapoyas province of Peru. Photo: Tiffany Kosch.

Dr. Scheele and his colleagues identified 501 species in decline, far greater than the previous estimate of 200. Certain factors once thought to account for the decimation of frog populations—like climate change and deforestation—are not the greatest threats, the scientists found.

“A lot of those hypotheses have been discredited,” said Dr. Scheele. “And the more we find out about the fungus, the more it fits with the pattern.”

As it turns out, Bd wiped out some species long before it was discovered. Only by going back to museum specimens were scientists able to estimate the toll. “It’s scary that so many species can become extinct without us knowing,” said Dr. Scheele.

The decimation of frogs peaked in the 1980s, the researchers found, a decade before the discovery of Bd. Today, 39 percent of the species that suffered population declines in the past are still declining. Twelve percent are showing signs of recovery, possibly because natural selection is favoring resistant animals.

As dire as the study’s results turned out to be, Dr. Scheele is guardedly optimistic about future wildlife outbreaks. The element of surprise may have had a lot to do with Bd’s devastating success.

“It wasn’t expected or predicted, and so it took the research community a long time to catch up,” said Dr. Scheele.

A lungless Ensatina salamander common on the West Coast. A fungus related to the one that threatens salamanders has wiped out entire populations of frogs. Photo: Tiffany Yap.

Averting a North American biodiversity crisis

In 2013, researchers discovered that a related fungus was attacking fire salamanders in Belgium. Called Batrachochytrium salamandrivorans (Bsal for short), it seemed poised to do to salamanders what Bd has done to frogs.

But this time, things are playing out differently.

A western red-backed salamander found near Portland, Oregon. Parts of North America are at risk of a fungal outbreak that is deadly to salamanders. Photo: Todd W. Pierson, University of Georgia.

U.S. Restricts Movements of Salamanders, for Their Own Good

Researchers discovered the outbreak and identified Bsal quickly. They immediately began running experiments to understand the threat it posed. Thanks to barriers to trade now in place, Bsal has yet to threaten another species anywhere.

“We’ve learned, and we’re dealing with it better,” said Dr. Scheele. “I guess the question is always, ‘Are we doing enough?’ And that’s debatable.”

There’s still plenty of reason to worry about outbreaks to come. Bd has yet to reach New Guinea, home to a wealth of amphibian species found nowhere else on Earth.

Mapping the threat of Bsal to North American salamanders.

(A) Bsal habitat suitability model based on 133 carrier occurrences and six bioclimatic variables. (B) Salamander species-richness map. (C) Salamander Bsal vulnerability model. Major ports (black squares) for salamander imports. Port 6 represents Mexico City. Read the full study here: Averting a North American biodiversity crisis.

If a Bd-infected frog got there—through the pet trade, or as an accidental stowaway—the fungus would have a vast number of vulnerable hosts to attack.

“It could be a meltdown of the ecosystems over there,” said Daniel Greenberg, a graduate student at Simon Fraser University and co-author of the Science commentary.

Giant Toxic Toads Move North in Florida

The loss of frogs can alter entire ecosystems.

Without tadpoles to guzzle algae, blooms may choke streams. Without frogs to eat insects, some disease-carrying species may become more common. Birds and other predators that eat frogs have to find alternatives.

Scientists are not even resting easy about the species that have emerged intact from the Bd assault. Another strain of Bd, or some different species of fungus altogether, may prove to be even deadlier.

Edited from the original New York Times article by Carl Zimmer in the “Matter” column.